Seismic Isolation Systems for Nuclear InstallationsIAEA-TECDOC-1905
Seismic Isolation Systems for Nuclear Installations
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IAEA-TECDOC-1905
IAEA-TECDOC-1905
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SEISMIC ISOLATION SYSTEMS
FOR NUCLEAR INSTALLATIONS
AFGHANISTAN ALBANIA ALGERIA ANGOLA
ANTIGUA AND BARBUDA ARGENTINA
ARMENIA AUSTRALIA AUSTRIA AZERBAIJAN BAHAMAS BAHRAIN BANGLADESH BARBADOS BELARUS BELGIUM BELIZE BENIN
BOLIVIA, PLURINATIONAL STATE OF
BOSNIA AND HERZEGOVINA BOTSWANA
BRAZIL
BRUNEI DARUSSALAM BULGARIA
BURKINA FASO BURUNDI CAMBODIA CAMEROON CANADA
CENTRAL AFRICAN REPUBLIC CHADCHILE CHINA COLOMBIA CONGO COSTA RICA CÔTE D’IVOIRE CROATIA CUBACYPRUS
CZECH REPUBLIC DEMOCRATIC REPUBLIC
OF THE CONGO DENMARK DJIBOUTI DOMINICA
DOMINICAN REPUBLIC ECUADOR
EGYPT EL SALVADOR ERITREA ESTONIA ESWATINI ETHIOPIA FIJIFINLAND FRANCE GABON GEORGIA
GERMANY GHANA GREECE GRENADA GUATEMALA GUYANA HAITI HOLY SEE HONDURAS HUNGARY ICELAND INDIA INDONESIA
IRAN, ISLAMIC REPUBLIC OF IRAQIRELAND
ISRAEL ITALY JAMAICA JAPAN JORDAN KAZAKHSTAN KENYA
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LAO PEOPLE’S DEMOCRATIC REPUBLIC
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NETHERLANDS NEW ZEALAND NICARAGUA NIGER NIGERIA
NORTH MACEDONIA NORWAY
OMAN
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PERUPHILIPPINES POLAND PORTUGAL QATAR
REPUBLIC OF MOLDOVA ROMANIA
RUSSIAN FEDERATION RWANDA
SAINT LUCIA
SAINT VINCENT AND THE GRENADINES SAN MARINO SAUDI ARABIA SENEGAL SERBIA SEYCHELLES SIERRA LEONE SINGAPORE SLOVAKIA SLOVENIA SOUTH AFRICA SPAIN
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THAILAND
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TURKEY
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GREAT BRITAIN AND NORTHERN IRELAND UNITED REPUBLIC
OF TANZANIA
UNITED STATES OF AMERICA URUGUAY
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VENEZUELA, BOLIVARIAN REPUBLIC OF
VIET NAM YEMEN ZAMBIA ZIMBABWE The following States are Members of the International Atomic Energy Agency:
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IAEA-TECDOC-1905
SEISMIC ISOLATION SYSTEMS FOR NUCLEAR INSTALLATIONS
INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 2020
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© IAEA, 2020 Printed by the IAEA in Austria
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IAEA Library Cataloguing in Publication Data Names: International Atomic Energy Agency.
Title: Seismic isolation systems for nuclear installations / International Atomic Energy Agency.
Description: Vienna : International Atomic Energy Agency, 2020. | Series: IAEA TECDOC series, ISSN 1011–4289 ; no. 1905 | Includes bibliographical references.
Identifiers: IAEAL 20-01308 | ISBN 978–92–0–106520–9 (paperback : alk. paper) | ISBN 978–92–0–106720–3 (pdf)
Subjects: LCSH: Nuclear facilities. | Seismic waves — Damping. | Earthquake resistant design.
FOREWORD
One of the statutory functions of the IAEA is to establish or adopt standards of safety for the protection of health, life and property in the development and application of nuclear energy for peaceful purposes. The IAEA is also required to provide for the application of these standards to its own operations as well as to assisted operations and, at the request of the parties, to operations under any bilateral or multilateral arrangement, or, at the request of a State, to any of that State’s activities in the field of nuclear energy.
This publication provides information on protection of nuclear installations against seismic events. It presents international practices and applications of seismically isolated systems that improve the seismic performance of structures, systems and components.
The methodology used to design seismically isolated systems has been tested and demonstrated to be effective in numerous non-nuclear seismically isolated buildings, bridges and other structures around the world, and several countries, including France and South Africa, have successfully constructed and operated seismically isolated nuclear installations. Moreover, in the light of the 2007 Niigataken Chuetsu-oki earthquake’s effects on the Kashiwazaki-Kariwa nuclear power plant, Japanese utility companies decided to build new base-isolated emergency buildings for each site. The behaviour and performance of these structures during the Great East Japan earthquake in 2011 confirmed the reliability of the design of these seismically isolated systems.
This publication supports the revision of IAEA Safety Standard Series No. NS-G-1.6, Seismic Design and Qualification for Nuclear Power Plants. The IAEA is grateful to all those in the international scientific community who contributed to the drafting and review of this publication. The IAEA wishes to thank P. Sollogoub for contributing to the drafting of the publication and A. Whittaker for comments and review. The IAEA officers responsible for this publication were O. Coman and N. Stoeva of the Division of Nuclear Installation Safety.
EDITORIAL NOTE
This publication has been prepared from the original material as submitted by the contributors and has not been edited by the editorial staff of the IAEA. The views expressed remain the responsibility of the contributors and do not necessarily represent the views of the IAEA or its Member States.
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iv CONTENT
1. INTRODUCTION ...1
1.1. Background ...1
1.2. Scope ...2
1.3. Objective ...2
1.4. Structure ...3
2. BASIC CONSIDERATIONS FOR APPLYING SEISMIC ISOLATION TECHNOLOGY ...4
2.1. Considerations on seismicity and site characteristics ...4
2.2. Considerations on horizontal and vertical isolation systems ...5
3. SAFETY CONSIDERATIONS ...5
3.1. Preventing failure modes of seismically isolated nuclear installations ...6
3.2. Monitoring of isolators characteristics variability ...7
3.3. Replacement prospect ...8
4. SEISMIC ISOLATION DESIGN ...9
4.1. Design codes and technical documents for seismic isolated systems ...9
4.1.1. Design codes for conventional seismically isolated structures ...9
4.1.2. Available technical documents for nuclear installations ...10
4.2. Design basis earthquake and input ground motions ...13
4.3. Dynamics of seismically isolated structures ...13
4.4. Types of isolation systems ...14
4.5. Elements of isolation devices ...15
4.6. Layout of isolators ...15
4.7. Analysis methodologies and modelling of a seismically isolated structures ...16
4.8. Analysis and design of seismic isolation systems ...17
4.9. Analysis and design of substructure and superstructure ...18
4.10. Analysis and design of internal SSCs ...18
4.11. Analysis and design of umbilicals ...19
4.12. Design allowable limits for the seismic isolation system ...19
4.13. Considerations for other external loads ...20
5. BEYOND DESIGN BASIS SEISMIC EVENTS ...21
6. SEISMIC PROBABILISTIC SAFETY ASSESSMENT FOR NUCLEAR INSTALLATION SEISMIC ISOLATION SYSTEMS ...22
7. QUALITY CONTROL AND MAINTENANCE OF ISOLATION DEVICES ...23
7.1. Technical requirements for design stage ...23
7.2. Technical requirements for procurement stage ...23
7.3. Technical requirements for construction stage ...24
7.4. Technical requirements for operation stage ...24
v
8. ECONOMIC CONSIDERATIONS ...25
9. INDEPENDENT REVIEW ...26
APPENDIX – Basic elements of isolation devices...29
A.1 Bearing devices...29
A.2 Dampers ...37
A.3 Three-dimensional isolation ...39
REFERENCES ...41
ANNEX I BASE ISOLATED NUCLEAR PROJECTS ...45
ANNEX II BASE ISOLATED EMERGENCY BUILDINGS CONSTRUCTED IN JAPAN NUCLEAR POWER PLANTS ...47
ANNEX III PERFORMANCE OF SEISMICALLY ISOLATED STRUCTURES TO SEISMIC EVENTS ...51
ANNEX IV COUNTRY REPORTS ...59
DEFINITION OF TECHNICAL TERMS ...105
ABBREVIATIONS ...107
CONTRIBUTORS TO DRAFTING AND REVIEW ...109
CONSULTANCY MEETINGS...109
1 1. INTRODUCTION
1.1.BACKGROUND
Seismic Isolation (SI) use has been growing over the last 20 years (Japan, USA, France, Italy).
The most common applications are on conventional (non-nuclear) structures, such as buildings, bridges, offshore oil and gas platforms, high hazard storage tanks, industrial facilities, etc.
Their design is based on developed codes and standards with controlled manufacturing and on- site construction procedures. These developments constitute the design and analysis techniques which are mature and reliable.
To date, the number of applications on nuclear installations is relatively small but growing.
Earthquake engineers around the world are considering base isolation systems to be applicable for nuclear installations as defined by the IAEA.
In principle, seismic isolation can be applied to new and existing nuclear power stations, processing facilities, and other nuclear facilities. Although it is easier to apply it to new structures, retrofit of complete structures and buildings with base isolation systems has been performed in the past. In existing facilities, seismic upgrade based on seismic isolation of a component, a system, or a sub-structure is also possible.
This publication relates to a number of IAEA Safety Standards, namely: SF-1 [1], SSR-1 [2], SSR-2/1 [3], SSR-2/2 [4], SSR-3 [5], SSR-4 [6], SSG-9 [7], NS-G-1.6 [8], NS-G-2.13 [9]. It complements these IAEA Safety Standards as a technical publication on the behaviour of seismically isolated SSCs in nuclear installations. Thus, this publication contributes to the implementation of IAEA Safety Standards by providing detailed technical basis in relation to seismic analysis, seismic design, and seismic safety evaluation; particularly, for the revision of Safety Guide NS-G-1.6 [8], Seismic Design and Qualification for Nuclear Power Plants.
The following paragraphs provide an overview of seismic isolation
a. The basic concept of seismic isolation is to filter out the medium and high frequency part of the seismic excitation applied to a building, a group of buildings, a component, a system, or a sub-structure.
b. This is achieved by adding flexible or sliding elements (isolators) between the structure to isolate and its support. The isolators shift the effective fundamental frequency of the isolated structure to low values, typically below 1 Hz, where the energy content of the seismic excitation is lower. It induces a reduction of the inertial forces and accelerations transferred to the structure and to the components and systems it may host.
c. As a consequence of lowering its effective fundamental frequency, the displacement response of the isolated structure relative to its support is increased. To limit this increase, damping devices are frequently integrated in the seismic isolation system.
d. Most of the existing isolation systems are effective in the horizontal directions only.
They provide flexibility in the horizontal direction while being relatively rigid in the vertical direction. The vertical rigidity is due to the fact that they bear the weight of the isolated structure, and that they do so without inducing large deflections or rotations of the isolated structure. The two main categories of isolators performing the above- mentioned tasks are the laminated elastomeric bearings and the sliding bearings, with multiple unique characteristics for each category.
2
e. Seismic isolation of structures or components in the vertical direction in addition to the horizontal directions is also feasible. This type of 3D isolation is generally achieved using appropriate devices with controlled dynamic stiffness in all three directions.
f. The expected increase in the seismic displacement response of the isolated structure implies specific design for the umbilical lines (distribution systems connecting isolated and not isolated parts; see Glossary).
g. The expected benefits of applying seismic isolation technologies for a nuclear power plant (NPP) or a nuclear facility are the following:
- Seismic acceleration response of isolated SSCs is reduced. This reduction can be necessary to justify the design or decrease the costs of some SSCs in moderate and high seismicity areas.
- Seismic design of SSCs can be standardized. It allows the installation of SSCs (with minimal design changes) on sites with higher seismicity than the standard design conditions.
- Generally, uncertainties in the response of a seismically isolated structure are lower than those of a non-isolated structure. This is because the response, in the isolated directions, is primarily dominated by the isolation system, where the dynamic behaviour is well known. The uncertainties in soil and building behaviour are of secondary importance.
1.2.SCOPE
This publication presents the current state of practice and uses of seismic isolation systems in nuclear installations. The scope of this publication is limited to passive isolation systems and therefore the methodologies and considerations discussed are not applicable to active or semi- active seismic isolation systems.
1.3.OBJECTIVE
This publication develops the technical basis for the use of seismic isolation systems in nuclear installations. The objectives of this TECDOC are to:
a. Provide technical basis to support the revision of IAEA Safety Guides for new design and re-evaluation of existing facilities to include seismic isolation;
b. Assemble technical elements to cover design, risk or margin evaluation, manufacture, construction, and operation activities;
c. Present basic technical considerations for base-isolated nuclear installations, and structures, systems and components (SSCs) as reflected by the current state of practice.
3 1.4.STRUCTURE
Section 2 presents the general considerations in application of seismic isolation; including seismicity, definitions, and a description of existing nuclear design codes.
Section 3 presents general safety considerations.
Section 4 concerns design of seismically isolated nuclear installations.
Section 5 concerns beyond design considerations.
Section 6 presents seismic safety assessment.
Section 7 pertains to Quality Control and maintenance of isolation devices.
Section 8 introduces economic considerations of seismic isolation in nuclear facilities.
In relation to the objectives of this publication, as proposed in Section 1.1, the technical bases to support the revision of IAEA Safety Guides are discussed in Sections 3 and 4. Technical considerations covering design, safety assessment, margin evaluation, manufacturing, construction and operation activities are presented in Sections 5 to 9 and Appendix A. The whole publication presents basic technical considerations for use of SI technology in nuclear installations.
The most important base isolated nuclear projects are listed in Annex I. Annex II of this publication describes the isolation system implemented in these buildings. Annex III presents some cases which summarize satisfactory behaviour, where the information for the Tohoku earthquake is taken from reference [10]. Annex IV presents activities in different countries - France, Germany and Russian Federation, Republic of Korea, and Japan - related to design, implementation and R/D of base isolation for nuclear installations.
4
2. BASIC CONSIDERATIONS FOR APPLYING SEISMIC ISOLATION TECHNOLOGY
There are many benefits of seismic isolation in design and construction of new nuclear installations:
Lower accelerations on structures and components, enabling simple, economical and standardised design.
Simple structural behaviour leading to a simplicity of the analyses – in some cases, static analysis may be applicable for equipment inside an isolated structure.
Increasing safety by decreasing uncertainties, due to the fact that the “critical” element is the seismic isolation system itself, for which the behaviour up to failure is better evaluated than that of a non-isolated structure.
Simpler layout, with possibly more slender buildings and more flexibility to locate equipment. For example, due to almost constant acceleration over the height, it is possible to have heavy or sensitive components located at higher elevations.
Cost reductions for new builds (in terms of scheduling and global price) due to the capability to reuse original design for middle range seismic input (typically 0.3g) and existing main components qualifications.
2.1.CONSIDERATIONS ON SEISMICITY AND SITE CHARACTERISTICS
As for any nuclear installation, site seismicity quantification is typically based on a good quality seismic hazard assessment, be it probabilistic seismic hazard assessment (PSHA) or deterministic seismic hazard assessment (DSHA). The site seismicity influences the amplitude, the frequency content and the duration of the earthquake signals affecting the structure. The amplitude of the site seismic response spectrum in the frequency range of the isolation system has a primary effect on the definition of the isolation system. It is worth noting that the seismicity characteristics, related to the low frequency content of the input signal, are different from the characteristics usually focusing attention for the design of a non-isolated structure;
this requires specific information, developed in Section 4.1. As a consequence, it is recommended to use site-specific Ground Response Spectra (GRS), and not “general” GRS, which may not be suitable for low frequency content.
Site geotechnical conditions play an important role for base isolated structures. Rock and hard rock sites are preferable for the implementation of seismic isolation. Soft soil conditions may be challenging because:
The performance of the isolation system may be decreased because of potentially high excitation at low frequencies and large induced displacements.
The potential for differential settlement is increased, which may influence the distribution of vertical loads on isolators and may result in a heterogeneous loading of the isolators, or alternatively, in a very thick lower basemat. Care needs to be taken during construction, in order to prevent unnecessary initial differential settlement.
Retention of soil pressure may result in thick walls to secure the space around the isolated building.
5 2.2.CONSIDERATIONS ON HORIZONTAL AND VERTICAL ISOLATION SYSTEMS Horizontal isolation alone is the most common base isolation of a structure. It can be provided by different types of rubber or sliding bearings, with a low horizontal stiffness but a high vertical one. Attention has to be paid to the fact that, the isolation system may not be effective in the vertical direction, but some vertical stiffness and damping may exist and could be taken into consideration. In addition, the vertical excitation may induce a response of the structure in all three directions which is comparatively more significant than for a non-isolated case (see Section 4.1). Rocking effects may also become more significant in comparison to the non- isolated case. It can also happen that the rocking motions be effectively increased by the isolation system itself, due to the non-rigid vertical stiffness of the isolators.
Vertical isolation can be provided by spring type systems coupled to guiding devices allowing only vertical movement of the isolated structure or component. Such systems can typically be applied to isolate equipment sensitive to vertical excitation only. Attention has to be paid to the potentially significant vertical deflection of such system if subjected to variation of the vertical load, including self-weight, during normal or accidental conditions.
3D isolation can be achieved either with each isolator acting in three directions, e.g. rubber bearings with low vertical stiffness, coil springs with separate dampers, or, with two isolation systems in series, the first one acting horizontally and the second one vertically. The use of 3D coil spring isolators was proposed for some new design of NPPs [11] and the use of two systems in series was proposed for fast breeder reactors [12] with a global horizontal isolation of the plant and a specific local vertical isolation of the reactor vessel. In case of implementation of a 3D isolation system, specific attention has to be paid to the potentially increased rocking effect, which may challenge the overall efficiency of the system.
3. SAFETY CONSIDERATIONS
Basic safety considerations for seismically isolated nuclear installations need to be consistent with Safety requirements of the IAEA for design, operation and site evaluation (IAEA SSR-1 [2],SSR-2/1 [3], SSR-2/2 [4], SSR-3 [5] and SSR-4 [6]). The implementation of a seismic isolation system adds a new system for which safety conditions are defined and the compliance to applicable safety requirements is demonstrated. An important aspect from a safety point of view is the fact that the base isolation system is a safety system which is not redundant as a whole. In addition, it is generally composed of a large amount of almost identical components;
the failure of one or a few of them is not allowed to influence the overall safety of the system.
However, the latter may be not applicable to the isolation of a small building or equipment.
The four functions a base isolation system needs to ensure are: i) vertical supporting function, ii) isolation function by accommodating the displacement by stiffness, iii) displacement control by damping, and iv) re-centering capability (cf. EN 15129 [13]). An inspection programme of the isolation devices is to be defined, and an associated monitoring programme established.
The base-isolated nuclear facility cannot be designed to be less safe and reliable than a non- isolated nuclear facility (both meeting the regulatory requirements and prescribed safety/performance goals for all design and beyond design cases). It needs to be robust enough and provide sufficient margin under design and beyond design conditions for earthquake and
6
other external events, such as fire, flood, tsunami, aircraft crash, internal or external explosions, etc. [14].
This implies the following:
The base isolation system safety requirements need to be translated to the isolation elements requirements and demonstration of compliance should be achieved.
The seismic isolation system and its supporting structures need to exhibit adequate seismic margins to failure, for all design and beyond design loading cases, in compliance with the safety requirements of the isolated structure.
The variation of characteristics of the isolation elements and of the isolation system need to be integrated into the design process and controlled at the manufacturing stage and throughout the operating life of the installation.
The feasibility of replacement and adjustment of one or more isolators needs to be ensured throughout the life of the plant and needs to be considered at the design stage. The replacement cannot damage the isolators to allow further inspections and tests.
It is expected that the seismic isolation system has a restoring capacity by design and will bring the isolated structure back, close to its initial position shortly (within a few minutes) after an earthquake, so that the isolation system and structure maintain their seismic resistance to aftershocks. This expectation is an explicit requirement in JNES and French documents (Annex IV), and implicit due to the type of isolators considered in the NUREG/CR document [11].
3.1.PREVENTING FAILURE MODES OF SEISMICALLY ISOLATED NUCLEAR INSTALLATIONS
Failure modes of a seismic isolation system are presented hereafter, including adequate means of prevention of these failure modes ( [15], [16] and Appendix A):
Base isolation system failure modes:
o Excessive displacement of rubber bearing isolators, with possible de- lamination of the bearing or rubber failure due to shear. This is prevented by determination of the failure limits of the bearings and implementation of design margin to this failure. It is possible to implement a hard stop in the isolation system design, so that failure due to excessive drift becomes geometrically impossible.
o Excessive displacement of sliding bearings, with possible contact of the slider with the external surface of the bearing. This is prevented by the implementation of adequate design margin to failure. It is possible to implement a hard stop in the isolation system design, so that failure due to excessive displacement becomes geometrically impossible.
o Buckling of isolators under combined vertical loads and horizontal drift. This is prevented by experimental determination of the buckling failure limit and implementation of design margin to this failure. Buckling can also be prevented by demonstration that no buckling of the bearing occurs before shear- compression failure.
7 o Excessive compression, with possible shear-compression failure of the rubber, or degradation of the contact surfaces of a sliding isolator. This is prevented by implementation of design margin to this failure.
o Excessive tension of isolator leading to shear-tension failure of the rubber. This may happen when overturning moment leads to significant tension in “corner”
isolators or because of vertical earthquake excitation. Different approaches can be adopted to prevent this failure:
o No tension is allowed to develop inside the bearings, either by requiring a minimum compressive stress in all design situations, or simply by allowing uplift to occur between the upper raft and the isolators. In the latter case, the consequences of the uplift need to be assessed and integrated into the design.
o Determination of the shear-tension failure limit of the isolator and implementation of design margin to this failure.
It is noted that sliding bearings do not allow uplift; therefore, their design needs to consider this situation if it occurs.
o Loss of bearing capacity due to an external event such as fire. This is prevented by using fire protected devices and/or avoiding fire sources near the base isolation system and by protecting the moat from external fires (e.g. in case of aircraft plane crash) with a specific structure (moat protective structure).
Umbilicals failure modes
Umbilicals are subjected to not only seismic acceleration but essentially to large support relative displacements during an earthquake. These large displacements may cause damage to umbilicals that are important to safety such as main steam-piping, cooling water/seawater piping, etc. To avoid this, either a specific layout is adopted in order to cope with differential displacements, or specific devices, such as angular expansion joints, may be included in the design.
Substructure failure modes
o Pedestal failures due to excessive loads transmitted to them.
o Excessive loads on the raft at the pedestal junction
Superstructure failure modes
o SSCs located in the superstructure are usually designed to remain elastic under design seismic loading. Failure modes are usually related to drift; therefore, adequate margins need to be considered for beyond design conditions
Isolated components
o Large displacements for low frequency equipment, such as fuel handling devices, are typically explicitly examined and their design modified if necessary.
o Sloshing of pools usually occurs at low frequencies and seismic isolation may increase sloshing loads.
3.2.MONITORING OF ISOLATORS CHARACTERISTICS VARIABILITY
Isolators’ mechanical properties have inherently more variability than conventional structural parts. Variability may arise during manufacturing (for example, rubber properties depend on
8
the vulcanization duration for rubber bearings or friction coefficients for sliding bearings).
Variabilities may also arise during construction (geometrical tolerances during installation of isolators affect the global properties of the isolation system), during operation (ageing process), during an earthquake, or other accidental loads (characteristics change due to the loading or due to cycling, for example lead plugs will heat up under repeated loading cycles and change characteristics). Variability due to ageing is controlled by in service inspection, as described below, and manufacturing and construction variabilities are managed by QA manufacturing and construction procedures, the variability due to loads and loading cycles is managed through test programs.
All types of base isolation systems exhibit, more or less, changes in mechanical properties with time (ageing process). These variations of properties need to be properly accounted for in the design by defining bounding values and performing the design analysis with these bounding values.
As a consequence (or additionally), it is necessary to monitor the mechanical properties of isolation system during the entire life of the facility, in order to check the ageing process and in order to confirm that the actual values remain within the bounding values assumed in design.
The management of ageing varies from member state to member state; it can involve a combination of accelerated ageing material tests, in-situ material tests and full-scale seismic isolators test.
For metallic parts, corrosion and relaxation needs to be monitored. For dampers, oil and similar products may be subjected to ageing and their characteristics are periodically tested and controlled.
3.3.REPLACEMENT PROSPECT
In order to cope with possible degradation of isolators, feasibility of replacement of one (or several) seismic isolation element(s) is usually required by Regulators. The need for such replacement could be the verification of properties of one or several systems to determine if properties are outside the design limits; the isolators have experienced accidental damages, an excessive deterioration of mechanical properties due to ageing or due to a strong external load –earthquake or other – or a change in seismic demand requiring new isolators. If the two latter cases are suspected and determined to be the cause of property changes, it is likely that almost all isolators may need to be replaced.
The replacement of one isolator needs to be considered in the design. As an example, in the French EDF NPP located in Cruas, replacement of two isolators was effectively carried out (AFCEN [17]) in the 90’s.
The replacement requirement in conjunction with inspectability, needs to be taken into account at an early design stage, as it will have an impact on the design of the substructure and the seismic isolation story. Moreover, the number of isolators located on a pedestal need to be limited and the distance between pedestals has to allow for inspection and replacement activities. In addition, it is often required [11] and [17] that the upper raft, with the appropriate load combinations, be designed for at least one missing support or isolator.
9 4. SEISMIC ISOLATION DESIGN
4.1.DESIGN CODES AND TECHNICAL DOCUMENTS FOR SEISMIC ISOLATED SYSTEMS
Design codes for conventional seismically isolated structures
Some design codes developed for conventional seismically isolated structures are presented in this subsection. They have been used as the basis for design of isolated nuclear structures as well. Seismic isolation elements covered by these codes are the same as those used in nuclear facilities, with the exception of the dimensions of the devices (generally larger for nuclear facilities).
ASCE/SEI 7-16 Minimum design loads and associated criteria for buildings and other structures, American Society of Civil Engineers (ASCE), 2016 -USA [18]
AIJ, Recommendation for the Design of Base Isolated Buildings, Architectural Institute of Japan, 2013. (in Japanese) [19]
JSSI ( [20], [21], [22]) Japan Society of Seismic Isolation developed texts giving list of possible devices, Guidelines for umbilical’s design and elements on maintenance for buildings and bridges.
EUROCODE 8:
EN 1998-1:2004 – Eurocode 8: Design of structures for earthquake resistance – Part 1:
General rules, seismic actions and rules for buildings [23]
EN 1998-2:2005 – EUROCODE 8: Design of structures for earthquake resistance – Part 2: Bridges [24]
These codes are used for conventional structures in Europe, together with the following standards, specifically dedicated to seismic isolation devices design:
EN 15129:2010 – Anti-Seismic Devices [13]
EN 1337:2005 – Structural bearings [25]
These codes are specifically adapted for the design of isolated nuclear facilities in France at RJH and ITER.
ISO22762 [26]: This international Standard is dedicated to elastomeric seismic isolators. Part 1 specifies the test methods for determination of the characteristics of elastomeric seismic isolators and for measurement of the properties of the rubber material used in their manufacturing. Part 2 describes applications for bridges and Part 3 is for building applications. This document presents a complete set of guidance for determination of isolators' properties (Low Damping Rubber Bearing (LDRB), High Damping Rubber Bearing (HDR), Lead Rubber Bearing (LRB), etc.) based on tests. Many independent national standards are based on this document.
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Available technical documents for nuclear installations
There are multiple general documents devoted to the seismic isolation of nuclear installations.
Only one, from Japan, can be considered as a design code, or more precisely, an application document for design:
JEAG 4614-2013, Seismic Design Guidelines for Base-Isolated Structures of Nuclear Power Plant, Japan Electric Association, 2013. [27]
The guidelines specifically describe design methods and procedures of seismic isolation such as calculation of design basis seismic force, design of isolation device, design of SSCs of base- isolated NPP, quality control, etc. The guidelines are based on JEAG 4614-2000 and revised to meet the requirements of the Regulatory Guide for Reviewing Seismic Design of Nuclear Power Reactor Facilities (Nuclear Safety Commission, 2006). In addition, the guidelines are revised considering the consistence of the design methods with Technical Code for Seismic Design of Nuclear Power Plants (JEAC 4601-2008, Japan Electric Association, 2008). This document is in Japanese, and no official translation exists at this time.
European Commission, Proposals for design guidelines for seismically isolated nuclear plants, EUR 16-559 EN, 1995. [28]
This is a first proposal for the development of guidelines for the use of seismic isolation for nuclear installations. A revision of the document was issued in 1998 in order to integrate 3D systems and rolling ball-dissipative layer systems, both developed in the Russian Federation.
The development of these two systems was discontinued. The initial document provides general information with few specific details, but does not provide much justification.
AFCEN French Experience and Practice of Seismically Isolated Nuclear Facilities The most recent designs of base isolated nuclear structures are the RJH research reactor and the ITER international project, both under construction in Cadarache, France. The design of these isolation systems did not rely on a single specific code but on a combination of the European codes described above and best practices developed by the French industry based on 30 years of return of experience in design, construction and monitoring of seismically isolated nuclear installations. The main elements of these practices and experiences have been included in the design document for Civil Works [17]. Annex A of Ref. [17] summarizes the main requirements of the document. It applies to the seismic isolation of a building or a complete installation. Some specific features are as follows:
• The material used for elastomeric isolation devices is synthetic rubber called poly- chloroprene (CR bearings), which has a long industrial history of manufacturing bearings in Europe.
• Higher margins are proposed for design as compared to the design of conventional non- nuclear structures. For instance, the maximum allowed seismic distortion is 1.4 for nuclear, compared to 2.5 for the conventional applications. Additionally, a minimum compressive capacity of 1 MPa is also required for non-anchored bearings.
• The effect of ageing on the mechanical properties of the isolators is determined based on measurements made on both, samples and actual bearings taken out from existing
11 isolated nuclear installations. These samples and actual bearings have experienced 30 years of ageing in actual environmental conditions.
• Specific requirements for quality control and maintenance of isolation devices are described for all stages of construction of the installation.
• Tolerances for the setting of isolation bearings are proposed based on the return of experience of the RJH and ITER projects.
• Some recommendations on the analysis methodology to generate floor response spectra are given in order to capture the possibility of significant peaks at higher mode frequencies.
U.S. Nuclear Regulatory Commission (USNRC) NUREG/CR-7253, Technical considerations for Seismic Isolation of Nuclear Facilities, 2018. [11]
This is a document which gathered technical elements on base isolation in order to prepare the drafting of a Safety guide on this subject by US NRC staff; it is not a regulatory document. The document addresses all relevant points, even if some parts are only briefly mentioned, (for example: structural analysis refers to other relevant documents in US, such as ASCE-4 [29] or ASCE-7 [18] or ASCE/43-05 [30]). The document develops a full performance-based and risk- informed design philosophy. The guidance mainly focuses on horizontal isolation of nuclear islands, composed of reactor building, nuclear auxiliary buildings and possibly other parts of the plant. Isolation of components is addressed to a limited extent. The main features addressed in the documents are as follows:
• The document addresses isolators common to US industry practice:
• Low damping natural rubber bearings
• Lead-rubber bearings
• Spherical sliding bearings
The document doesn’t cover other types of bearings used in different countries, such as high damping rubber, synthetic rubber, or 3-D isolation; these systems are only acknowledged, despite the fact that some are used extensively in other countries.
• From a safety analysis point of view, specific points are related to the particular situation of the isolation system, which is a non-redundant safety related system.
Therefore, it needs to have more stringent design criteria than more conventional construction. The isolators cannot be allowed to fail and need to be removed from any realistic sequence of potential failure of the plant due to earthquake shaking.
• The potential for cliff edge effects is to be removed through the use of a hard stop.
• Recommendations for the design of the moat gap are suggested.
• A passive re-centring system can be included.
• Performance criteria are proposed based on performance-based approaches described in ASCE 43-05 with some adaptations. In addition to the Design Basis Earthquake, for
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which the same criteria as for non- isolated plants are applied, an Beyond Design Basis earthquake is defined with a 100,000-year return period. It is to be at least equal to 1.67 times the Design Ground motion. For the Beyond Design Basis Earthquake, ultimate requirements for the isolation system are proposed in Table 8-1 of Ref [11].
• Three options for structural analysis are mentioned: 1) coupled time domain, 2) coupled frequency domain, and 3) multi-step. Coupled 3D time domain modelling has no usage restrictions, coupled frequency domain can only be used with low damping rubber bearings (essentially linear) without damping and, in certain limited circumstances, to provide input to the multi-step method.
• Tension or uplift of the superstructure relative to the isolators is allowed, provided that their effects are correctly taken into account.
• Assurance of performance needs to incorporate a combination of prototype and production testing to physically demonstrate quantifiable confidence levels and performance reliability in both the isolators and the umbilicals.
ASCE Standard, ASCE 4-98 Seismic Analysis of Safety-Related Nuclear Structures and Commentary. ASCE, 1998
This document is the basis for the analysis of nuclear structures in many countries. There are very few specific elements for base isolated nuclear structures: some very general elements are provided in Section 3.5.6. The revision of this document includes more specific data for Isolated Structures, with a performance based approach coherent with ASCE 43-05.
ASCE Standard, ASCE 4-16 Seismic Analysis of Safety-Related Nuclear Structures and Commentary. ASCE, 2017 [29]
This document is the revised version of ASCE 4-98 and includes guidance specific to seismically isolated structures in Section 7.7.
JNES, Seismic Safety Division, Proposal of technical review guidelines for structures with seismic isolation, report n° JNES-RC-2013-1002. [31]
This report is the first complete edition of a document which was drafted by JNES, the former Technical Support Organisation to the Japanese safety authority and presently integrated inside Nuclear Regulation Authority (NRA) to give guidelines for the review of projects of seismically isolated nuclear installations. It is specifically intended to be usable by different countries, covering a large variety of installations in low to high seismicity regions. The document covers building and floor or equipment isolation. For each subject, the document defines the principles and provides a commentary which gives more detailed information.
There are few numerical prescriptions, but more general definitions of requirements for which specific values are defined by the designer. Some features of this document are:
• The document does not recommend a specific type of isolation system, but general indications are given in order to define criteria for each system
• New and existing facilities are considered; for the latter, only equipment or floor isolation is suggested
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• Horizontal and vertical systems are included
• Seismic structural analysis methods are similar to non-isolated ones
• Beyond design and margin considerations are treated in the framework of residual risk assessment, following the 2006 Japanese seismic regulatory document. No additional guidance is proposed.
• The entire plant life is covered, including the definition of tests and inspections during the pre-operation phase, operation phase and ageing management, and performance tests after an earthquake.
• The document is supplemented by examples of seismic isolation trial design and preliminary assessment for nuclear buildings (Pressurized Water Reactors (PWR) and Boiling Water Reactors (BWR)), equipment isolation (computer system, floor supported system), design of connecting piping systems and design of equipment in a BWR. In addition, some papers on fragility estimation of components are included.
An abridged version of the document is included in the Annex IV of this publication.
4.2.DESIGN BASIS EARTHQUAKE AND INPUT GROUND MOTIONS
The Design Basis Earthquake needs to be established per IAEA Safety Standards with appropriate considerations of local site conditions.
For seismically base isolated structures, the following elements are considered explicitly in the development of a Design Basis Earthquake, which is typically represented by a ground motion response spectrum and/or ground motion time series.
- A range of frequencies that includes the effective period of the isolation under a ground motion representing the design basis earthquake with appropriate margins. The effect of velocity pulses that may result from large magnitude earthquakes at small site-to- source distances ( [32], [33])
- The duration of strong ground motion, which affects the response of nonlinear isolation systems sensitive to cycling loads. It should be noted that high magnitude earthquakes far from the site generally have a very long duration [34].
If three-component acceleration time series are used for analysis, it needs to be ensured that the records are consistent with the Design Basis Earthquake. Information on the generation of time series can be found in IAEA Safety Guide SSG-9 [7], NIST GCR 11-917-15 [35], ASCE 4-16 [29], and Japanese NRA Regulatory Requirements [36].
4.3.DYNAMICS OF SEISMICALLY ISOLATED STRUCTURES
The seismic response of an isolated structure can be described by the following elements:
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In the directions of isolation, assuming linear behaviour of the isolation system, the isolated structure response is dominated by its first mode. For an efficient isolation system, this first mode corresponds to a deformation of the isolation story and a quasi- rigid translation of the superstructure.
For nonlinear horizontal isolation systems, the effective isolation period varies as a function of horizontal displacement. The response of the superstructure is a function of the hysteretic characteristics of the isolation system.
For linear horizontal isolation systems, the horizontal acceleration response of a stiff superstructure is approximately constant if the effects of rocking are small. The horizontal response in a given direction is roughly equal to the horizontal spectral acceleration in that direction at the frequency and damping of the isolation system. The displacement response in a given direction is approximately equal to the horizontal spectral displacement in that direction at the frequency and damping of the isolation system.
For nonlinear isolation systems, the use of a ground response spectrum is not appropriate to predict the response acceleration and displacement of the isolated structure, even if it can give a first order of magnitude. For such systems, nonlinear response-history analysis is to be used to compute the response of the superstructure.
For the purpose of analysis, three-component time-history ground motions need to be selected and scaled to be consistent with the input spectrum. The response is significantly dependent on the characteristics of these time histories, such as the strong motion duration, or presence of a velocity pulse at low frequencies ( [32], [33]).
The isolators of the isolation system are typically modelled explicitly in the mathematical model to capture the effects of torsion and rocking of the superstructure.
The isolation system translates and rotates in response to the seismic inputs and the distributions of mass and stiffness in the isolation system and superstructure. The effects of torsion and rocking on the isolators is greatest at the periphery of the isolation system. The torsional response can be mitigated by the placement of the stiffest isolators at the perimeter of the isolation system. Rocking of an isolated structure, if important, can be mitigated through the addition of vertically stiff damping devices. It can be taken into account that an increase in damping will decrease the relative displacement between isolated and non-isolated parts but may have detrimental effects on the superstructure response ( [37], [38]).
The three-dimensional response of the isolation system and isolated superstructure need to consider all three components (two horizontal and one vertical) of seismic input. The geometry of the isolated superstructure may result in coupling of horizontal and vertical modes of response in the superstructure [39].
4.4.TYPES OF ISOLATION SYSTEMS
The choice of an isolation system is a function of the seismic demand (see Design Basis Earthquake in Section 4.2), site conditions (soil, temperature, environment, etc.), weight of the isolated structure, and its expected seismic response. The choice needs to rely on industry experience in the country of application and in the country of manufacturing of the isolators.
15 The type of isolation elements that can be assembled to form a seismic isolation system is chosen based on the loading conditions at each of the isolators.
Except for active and semi-active isolation systems (see Section 1.2), there is no restriction to the type of technology that can be used, provided that it meets the safety requirements and that its characteristics are fully determined by an appropriate test program, including quantification of all possible variabilities.
4.5.ELEMENTS OF ISOLATION DEVICES
This section provides basic information about seismic isolation elements used or considered in the design of nuclear installations. These elements, alone or assembled, constitute the seismic isolation device which provides the needed isolation function. Table 1 presents, in a simplified way, a possible classification of isolation elements. There are two categories of isolation devices: the first one is bearing devices, which generally have the function of bearing (supporting the weight of the isolated structure or equipment) and of filtering seismic excitation, the second category is dampers, which have the function of adding damping to the system in order to limit the drift of the isolation system.
A large variety of devices exist. All are not suitable for nuclear installations which are safety related and required to have a long design life. In addition, certified materials are typically used for isolation devices. A detailed description of isolation devices is presented in Appendix A.
TABLE 1. CLASSIFICATION OF BASE ISOLATION DEVICES
Bearing Devices and Elements
Laminated Rubber Seismic Isolation elements:
A. Low Damping Rubber Bearing B. Lead Rubber Bearing
C. High Damping Rubber bearing Sliding elements
A. Rigid Sliding Bearing B. Elastic Sliding Bearing C. Friction Pendulum System Coil Springs
Other
Dampers 1D, or Multi-Directional
Hysteretic Damping type A. Steel damper B. Lead damper Viscous damping devices
A. Oil devices B. Viscoelastic type 4.6.LAYOUT OF ISOLATORS
The following considerations establish the design of the isolators’ layout:
a) It is recommended that vertical loads are homogeneously distributed on the bearings (Ref. [17], [23] and [24] specify an allowable variation of loads between different isolators of ±20%). This limits the flexural stresses in the basemat and differential behaviour between isolators that might depend on the vertical load (for example,
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friction forces). Soil settlement is to be considered when assessing the distribution of vertical loads on the bearings.
b) Offset between the rigidity centre of the isolation system and the gravity centre of the superstructure (eccentricity of the seismic isolation system) is to be limited to a value as low as reasonably achievable, in order to minimize the torsion phenomenon. The torsion phenomena can also be mitigated by the placement of stiffer isolators at the perimeter of the isolation system.
c) Rocking motions of an isolated structure need to be considered as part of the layout design. If significant rocking could be mitigated through addition of vertically stiffer damping devices at the periphery of the basemat.
d) To prevent load transfer through the basemat (which complicates its design and increases the uncertainty in calculation of the bearing reaction loads and reduces the potential of rocking induced by vertical excitation), isolators can be located directly below the vertical structural elements: walls or columns, where possible and achievable.
e) The layout needs to ensure the ability to inspect and replace any of the individual isolators. For isolated buildings, this consideration implies sufficient space all the way around, or at least on one side of the isolators, to support replacement activities.
Circulation paths through the inter-basemat space are also to be arranged to allow for insertion and removal of isolators and tools necessary to perform the replacement.
4.7.ANALYSIS METHODOLOGIES AND MODELLING OF A SEISMICALLY ISOLATED STRUCTURES
Analysis methods for seismically isolated structures are basically the same as those used for non-isolated ones [40]. Some points requiring specific attention are summarized hereafter:
a) The superstructure is usually represented by a 3D model with sufficient details to capture rocking and torsional motions of the superstructure as well as any other local coupling effect between the directions of excitation (as highlighted in Section 4.3).
b) For non-linear isolation systems, response evaluation of the base-isolated structure is typically performed by time history analysis, accounting for the system’s non-linear behaviour. The excitation is typically applied simultaneously in the 3 directions.
c) For linear isolation systems with low damping, modal response spectrum methods can be used. Time history analysis, possibly by modal superposition, remains the preferred method, at least to generate in structure response spectra. The excitation is typically applied simultaneously in the 3 directions.
d) The model usually allows for a precise evaluation of the reaction forces and of the possible uplift on each isolator individually. Therefore, the positioning of the isolators below the superstructure in the model needs to be realistic.
e) The isolators’ properties inserted into the analysis model such as force-displacement relationships (horizontal and vertical) and/or damping properties are always based on representative tests of the products.
17 f) The variability of the isolators’ properties due to ageing, loading, cycling, temperature, environment condition, manufacturing, installation or others, need to be either explicitly included into the analysis model or covered by performing boundary cases analyses in the design process.
g) Soil-structure interaction effects are usually included into the model, except if demonstrated to be insignificant. These effects are generally negligible for an isolated structure on a hard rock site, though only in the directions of isolation.
h) Attention should be paid to the modelling of the superstructure structural damping in case of performing non-linear time history analyses. In particular, the use of a complete Rayleigh damping for the superstructure is incorrect, because the damping term resulting from the mass matrix would spuriously damp the “rigid body” motion of the structure on its isolation system. Possible alternatives are to use modal damping for a linearly modelled superstructure or to develop a specific damping matrix applied to the velocity of the superstructure relative to the basemat [41].
i) For analyses using in-structure response spectra, the horizontal and vertical seismic loads are typically combined by appropriate methods considering the vibration characteristic of the base-isolated structure. SRSS is not always appropriate for modal combination because, very low and medium frequency signals are cumulated, for which maxima generally cannot be considered as independent, and because medium frequency responses can be in-phase and need to be cumulated algebraically. In general, Newmark combination for directions is acceptable.
4.8.ANALYSIS AND DESIGN OF SEISMIC ISOLATION SYSTEMS
The design of the seismic isolation system typically ensures that the allowable design limits are respected, with due consideration of the variability of the isolators’ characteristics, when applying the design input ground motions to one or several analysis model(s) of the structure [42]. Details of the analysis and design process for the seismic isolation systems are presented in the following sections of this TECDOC:
- Section 4.2 for the definition of the Design Basis Earthquake and the generation of the input ground motions
- Section 4.3 for an overview of the dynamics of a seismically isolated structure - Section 4.7 for the appropriate analysis and modelling methodologies
- Section 4.12 for the definition of the design allowable limits for the isolation system One important characteristic of almost every base isolation system is its nonlinear behaviour:
sliding, non-linear elasticity, non-linear damping behaviour, etc. As a result, linear analyses approaches, such as conventional modal response spectrum methods, are not directly applicable, at least without some adaptation for analyses of structures, isolation systems, and for floor response spectra derivation. In addition, non-linear behaviour requires the use of more time-histories than a conventional linear time-history approach. Simplified approaches, such as equivalent linearization, may be applied in some cases; however, they need to be carefully validated. Another question raised by this fact is, for example, that linear combination is not
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strictly applicable, for extrapolation of results with a stronger input. Specific attention should be paid to design representation of non-linear damping and its computation [43].
4.9.ANALYSIS AND DESIGN OF SUBSTRUCTURE AND SUPERSTRUCTURE
As for any other structure, the substructure and the superstructure are to be designed based on recognized codes and standards. It should be noted that some topics, specific to the design of seismically isolated buildings require special care:
The lateral walls of the substructure, bearing soil pressure loads in normal and seismic conditions, is to be designed with a reliability that is at least equal to that required for the seismic isolation system itself (Section 3.2).
The pedestals or walls supporting the isolators is to be designed with a reliability that is at least equal to that required for the seismic isolation system itself. Seismic capacity of the substructure and isolators system is to be coherent and adequate capacity design is to be applied.
The superstructure is to be designed to remain in its elastic range for the design basis earthquake and, as far as reasonably achievable, for the beyond design earthquake loadings as well. Because of the low frequency content of the excitation transmitted by the seismic isolation system, the ductility of the superstructure does not provide margins comparable to the ones obtained for a non-isolated structure ( [44], [45]). This point is only indirectly mentioned in seismic codes for conventional structures by limiting the behaviour coefficient of the superstructure to a value close to 1. For nuclear installations, ductility coefficients are to be considered with extra care. Furthermore, an additional margin can be applied to SSCs. Many nuclear power plant projects are basically designed for minimum seismic conditions (around 0.25g-0.30g ground acceleration), with the possibility of base isolation for sites with higher seismicity. This design procedure provides generally sufficient margins to cope with the increased ductility demand.
4.10. ANALYSIS AND DESIGN OF INTERNAL SSCS
The analysis and design of equipment and other SSCs installed in an isolated structure typically follow procedures used for non-isolated structures, recognizing that the use of an isolation system will alter the shape and ordinates of floor response spectra and the frequency content of floor input signals. Therefore, coefficients used in modal analysis of an isolated SSC with complex quadratic combination (CQC) method, are adapted to be consistent with the isolated structure.
Although the use of isolation will generally substantially reduce horizontal spectral demands [16], longer period parts of equipment such as arms on fuel handling machines may experience greater demands and re-qualification may be required.
Qualification may involve an earthquake simulator and significant long period spectral demands may be difficult to achieve in commercial test facilities. Thus, supplemental methods may be required to demonstrate adequate long period capacity. If the pseudo acceleration value of the floor response spectra is below 1g, static testing of the equipment with an angle to the vertical axis could replace a dynamic test.
